Move `createCredentials` to `tls` module and rename it to
`createSecureContext`. Make it use default values from `tls` module:
`DEFAULT_CIPHERS` and `DEFAULT_ECDH_CURVE`.
fix#7249
Commit 03e008d introduced src/tls_wrap.cc and src/tls_wrap.h but
said files copied on the order of 1 kLoC from src/node_crypto.cc
and src/node_crypto.h. This commit undoes some of the duplication.
Fixes#6024.
Split `tls.js` into `_tls_legacy.js`, containing legacy
`createSecurePair` API, and `_tls_wrap.js` containing new code based on
`tls_wrap` binding.
Remove tests that are no longer useful/valid.